The technical sector considered is that of polymers or copolymers used to improve the rheological performance of engine lubricating oils within their operating temperature range.
It is known that a formulated oil must have as high viscosity as possible at high temperature and as low viscosity as possible at low temperature. This characteristic is measured by the selectivity of an additive, defined as the relationship between the relative variation of the kinematic viscosity of the lubricating oil provided by the additive at high temperature (for example, 100.degree. C.) and the relative variation of the kinematic viscosity at lower temperature (for example, 30.degree. C.).
The selectivity of an additive depends in general on its composition or structure and its thickening power and therefore its molecular weight. A high thickening power makes it possible to formulate an oil with a smaller quantity of additive but only in so far as its mechanical shearing strength remains compatible with the application contemplated. Furthermore, the formulated oil must retain a low pour point. Various types of additives are known which meet a lesser or greater number of the criteria stated above. The performance for alkyl polymethacrylates in regard to temperature is good (selectivity between -18 and 150.degree. C. of about 2). They lower the pour point of the oil to about -33.degree. C., but their thickening power is limited by their mechanical shearing strength. Quantities on the order of 4 to 6% of additives are required to adjust the viscosity of the oil without the formulation becoming too sensitive to shearing. This is reflected by a higher usage cost of these additives than that of other additives of the hydrogenated diene-styrene copolymer type or of the ethylene-propylene copolymer type, for example, which has a higher shearing strength but a definitely lower selectivity than polymethacrylates (0.7 to 0.9 measured between -18.degree. and 150.degree. C.). Furthermore, the performance in cold conditions of these additives is not satisfactory and requires the addition of some anti-freeze additive, which is not necessary with viscosity additives of the polymethacrylate type.
Copolymers are also known which aim at combining the characteristics in solution of various homopolymers. These copolymers are made by grafting polymethacrylate onto a backbone of an olefinic copolymer or diene-styrene copolymer and improve the stability of the mixtures of olefinic polymers with freezing-point lowering additives of the polymethacrylate type (see U.S. Pat. No. 4,290,925). In practice, these techniques are difficult to apply and require the separate preparation of polymers and a mixing or emulsifying operation.
In addition, free-radical grafting techniques are rarely quantitative and there is always the risk of their leading to partially cross-linked products which would no longer be soluble in the base oils.
Styrene/alkylmethacrylate block copolymers are also known, which are prepared by the ionic pathway (see U.S. Pat. No. 4,136,047); they have a greater shearing strength than polymethacrylates. However, anionic copolymerization requires precautions to be taken which make its industrial use difficult. In particular, the operations must be carried out in the absence of oxygen and humidity.
Styrene/alkyl methacrylate copolymers are further known which are prepared by the free-radical way and have high styrene contents (35 to 55% by weight). To retain sufficient solubility, these products have a C.sub.16 to C.sub.20 alkyl methacrylate content of at least 80% by weight of the total methacrylate content (see European application No. 0,056,342).
They have a greater shearing strength than polymethacrylates and are compatible with styrenic polymers. However, they have appreciably lower selectivity than conventional polymethacrylates.
Finally, German Pat. No. 1,122,197 may be pointed out, which discloses additives for lubricating oils consisting of copolymers formed of 55 to 90% by weight of C.sub.10 to C.sub.18 alkyl acrylates, 5 to 45% by weight of C.sub.1 to C.sub.4 alkyl acrylates or methacrylates and 2 to 20% by weight of styrene (or substituted styrene). Because of the presence of acrylates, these polymers have too high a sensitivity to oxidation and insufficient thermal stability.